The most common and most likely answer to this crossword clue is the 6 letter word CRUNCH
I've seen this crossword clue in The Daily Telegraph
C
R
U
N
C
H
Buddy explains!
The straight part
We know that "Champ/moment of truth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case champ/moment of truth = crunch.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CRUNCH mean?
CRUNCH noun- A noisy crackling sound; the sound usually associated with crunching.
- A critical moment or event.
- A problem that leads to a crisis.
- (exercise) A form of abdominal exercise, based on a sit-up but in which the lower back remains in contact with the floor.
- The overtime work required to catch up and finish a project, usually in the final weeks of development before release.
- A dessert consisting of a crunchy topping with fruit underneath.
- (generally in the plural) A small piece created by crushing; a piece of material with a friable or crunchy texture.
- To crush something, especially food, with a noisy crackling sound.
- To be crushed with a noisy crackling sound.
- To calculate or otherwise process (e.g. to crunch numbers: to perform mathematical calculations). Presumably from the sound made by mechanical calculators.
- To grind or press with violence and noise.
- To emit a grinding or crunching noise.
- To compress (data) using a particular algorithm, so that it can be restored by decrunching.
- To make employees work overtime in order to meet a deadline in the development of a project.